I dont use joomla, but I assume you may need to edit the config file.
or add something to your .htaccess file that rewrites http to http.
something like this:
# https to http.
Options +FollowSymLinks
RewriteEngine On
RewriteCond %{HTTPS} on
RewriteRule (.*)
http://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
this might or might not work for you.
if it causes problems simply remove the above code.
Note: the proper way is to change the https to http in joomla.
this is only suggested as a temp fix that might help.